1. Field of the Invention
This invention relates generally to over-the-road semi-trailers and, more specifically, to a system for reducing aerodynamic drag incurred by the shape and speed of the trailer.
2. Prior Art
Over-the-road semi-trailers are usually shaped like rectangular solids. This shape maximizes the loadable volume of the trailer for its size. It has long been known that shape causes substantial aerodynamic drag, which increases with the speed of the trailer, and increases fuel consumption. This drag was not a significant economic problem when gasoline and diesel fuel were inexpensive. However in recent years, fuel prices have skyrocketed and continue to be quite unstable. Thus, reduction in drag has a significant impact on gas mileage of tractor-trailers, which affects operation costs and, ultimately on profitability.
This drag has two components. One is the headwind resistance caused by the large flat front of the trailer impacting the air mass as it moves through it. For this reason, many commercial shippers mount a bubble on the upper front of the trailer, or provide the tractor with a wind deflector. The other component is rear turbulence, caused by the flat rear of the trailer, which causes eddying of the air as it passes around the trailer.
Many attempts have been made to reduce this eddying, most notably the addition of a xe2x80x9cbustlexe2x80x9d to the rear of the trailer. This approach can be seen in U.S. Pat. No. 5,375,903xe2x80x94Lechner, U.S. Pat. No. 4,978,162xe2x80x94Labbe, and U.S. Pat. No. 5,236,347xe2x80x94Andrus, for example. All of these devices are bulky, cumbersome and expensive. They effectively reduce the length, and necessarily carg capacity, of the trailer in states where there are length restrictions. None of them have been commercially successful.
Another approach has been to duct air from the front of the trailer through ducting within the trailer and eject this air at the rear of the trailer. This approach dates back nearly a century in application to rail cars, as exemplified by U.S. Pat. No. 926,971xe2x80x94Ahrens. More recent examples of this approach applied to semi-trailers are seen in U.S. Pat. No. 5,782,521xe2x80x94Anderson, and U.S. Pat. No. 2,514,695xe2x80x94Dempsey. In all of these, the ducted aor occupies internal volume of the trailer, necessitating a reduction in cargo capacity.
Thers is a need for a system of reducing drag for a trailer that is simnple, and does not reduce the cargo capacity of the trailer.
It is therefore an object of this invention to provide a system of improving gas mileage of a tractor-trailer by reducing drag, that is simple, and does not reduce the cargo capacity of the trailer.
In one aspect, this invention features a system for reducing the drag on a trailer, comprising air ducting mounted beneath the trailer which has an intake located beneath the trailer, and an outlet located on the flat rear surface of the trailer, whereby movement of the trailer forces air to flow into the intlet, through the ducts and out the outlets. Preferably, the duct inlets face the front and the side of the trailer, and the ducting forces all inlet air into a common plenum, through a venturi and out through a series if outlets spaced about the rear surface of the trailer.